The present invention generally relates to a position measuring system, and more particularly to a system for measuring a position of an object moving in a limited area, the position measurement being done by detecting pseudo-noise signals from a transmitter carried on the object by means of a plurality of stations provided in the area, so that coordinates of a detected position of the object are calculated and output. This system is applicable to local-area radiocommunication, factory automation, home automation or the like.
In the prior art, there exists a position measuring system for measuring a position of an object moving in a limited area, the area being located indoors in particular. In this system, a plurality of stations is provided in the area for receiving pseudo-noise (PN) signals from a transmitter carried o the object. The coordinates of a position of the object are calculated by a computer from the received signals. Such a system is disclosed, for example, in Japanese Laid-Open Patent Application No. 3-205579. In the system, the object carries a transmitter for transmitting the pseudo-noise (PN) signals to the stations, and it is necessary that the object has a clock coupled to the transmitter for sending sync signals as a time base for controlling the timing of transmission of the PN signals by the transmitter. The plurality of stations is connected to the computer via connecting cables and a second clock is coupled to the computer, so that correlative pulses are sent from the stations to the computer and the coordinates of the position of the object are calculated from the received correlative pulses in accordance with the time base established by the second clock. Also, it is required that each of the stations has a synchronizing circuit for controlling the timing of reception of the PN signals from the transmitter and the timing of sending of the correlative pulses to the computer. According to the conventional system, it is possible to carry out real-time tracking of the position of the object moving in the limited area. However, there is a problem in that the transmitter carried on the object requires a clock and in that each of the stations requires a synchronizing circuit. The structure of the system thus becomes complicated and the manufacturing cost is increased.
In the prior art, a modified position measuring system has been proposed for use in a case where it is not necessary to perform a continuous tracking of the position of the object when the object moves slowly in the area or the movement of the object is relatively small. The modified position measuring system has a simple structure and can periodically detect the position of the moving object in the area. In the modified system, a matched filter for generating correlative pulses in response to the received PN signals is used instead of the synchronizing circuit as described above. However, in the modified system, time durations between transmission of the PN signal by the transmitter and reception of the same by the stations are measured, and the coordinates of the position of the object are calculated by the computer from the measured time durations. Thus, it is also necessary that the measurement of the time duration and the calculations of the coordinates of the position are done according to the time base established by the second clock coupled to the computer. Moreover, there is a problem in that the measured time durations may have different variations between the stations due to the transmission delay of signals in each the connecting cable between the computer and the stations. Thus, the calculated coordinates of the position of the object may have significant errors.